Final answer:
a) Defibrillation is the most crucial early intervention for a patient in cardiac arrest with a shockable rhythm on the cardiac monitor, which can help restore a normal heartbeat.
Step-by-step explanation:
The most important early intervention when EMS personnel arrive to find a patient in cardiac arrest with bystanders performing CPR is defibrillation, if the cardiac monitor indicates a shockable rhythm such as ventricular fibrillation or ventricular tachycardia. The application of a large shock of electrical energy can terminate the arrhythmia and allow the body's pacemaker to resume normal patterns. While CPR provides vital support by circulating blood and oxygen when the heart is not capable of doing so effectively, the use of an automated external defibrillator (AED) as soon as possible is imperative to attempt to restore a viable cardiac rhythm in the case of certain life-threatening arrhythmias.